Distance estimation

ABSTRACT

A method, comprising: periodically enabling reception of a signal at a receiver, every first time; transforming the received signal in order to determine data in the received signal; comparing the determined data with reference data; and using the difference between the determined data and the reference data to estimate a correction to a multiple of the first time in order to determine a distance between the receiver and an origin of the signal.

FIELD OF THE INVENTION

Embodiments of the present invention relate to distance estimation. In particular, they relate to apparatus, a method, a computer program, a chipset and a module for distance estimation using at least one radio frequency signal.

BACKGROUND TO THE INVENTION

In many situations, it is desirable to determine the distance from one point to another, for example, to locate an object. It is possible to determine a distance between two points by using radio frequency (RF) waves. Some methods of distance determination involve using a first device to transmit an RF signal to a second device, and determining the distance between them by analyzing the attenuation that has occurred during the propagation of the signal.

Other methods of distance determination involve determining the time of flight of a signal that is transmitted from a first device to a second device and then using the equation:

d=c×t _(tof)  (1)

where d=the distance between the first and second devices, c is the speed of light and t_(tof) is the time of flight.

In order to make estimate the distance as accurate as possible, the time of flight should be determined with as little error as possible.

BRIEF DESCRIPTION OF VARIOUS EMBODIMENTS OF THE INVENTION

According to various embodiments of the invention there is provided a method, comprising: periodically enabling reception of a signal at a receiver, every first time; transforming the received signal in order to determine data in the received signal; comparing the determined data with reference data; and using the difference between the determined data and the reference data to estimate a correction to a multiple of the first time in order to determine a distance between the receiver and an origin of the signal.

According to various embodiments of the invention there is provided apparatus, comprising: a receiver configured to be enabled periodically to receive a signal, every first time; transformation circuitry configured to transform a received signal, to determine data in the received signal; a comparator configured to compare the determined data with reference data; and estimation circuitry configured to use the difference between the determined data and the reference data to estimate a correction to a multiple of the first time, in order to determine a distance between the receiver and an origin of the signal.

According to various embodiments of the invention there is provided a module, comprising: transformation circuitry configured to transform a received signal, to determine data in a received signal, the signal being received periodically every first time; a comparator configured to compare the determined data with reference data; and estimation circuitry configured to use the difference between the determined data and the reference data to estimate a correction to a multiple of the first time, in order to determine a distance between the receiver and an origin of the signal.

According to various embodiments of the invention there is provided a chipset, comprising: transformation circuitry configured to transform a received signal, to determine data in a received signal, the signal being received periodically every first time; a comparator configured to compare the determined data with reference data; and estimation circuitry configured to use the difference between the determined data and the reference data to estimate a correction to a multiple of the first time, in order to determine a distance between the receiver and an origin of the signal.

According to various embodiments of the invention there is provided a computer program, comprising: instructions for transforming a received signal, to determine data in a received signal, wherein the signal is received periodically every first time; instructions for comparing the determined data with reference data; and instructions for using the difference between the determined data and the reference data to estimate a correction to a multiple of the first time, in order to determine a distance between the receiver and an origin of the signal.

BRIEF DESCRIPTION OF THE DRAWINGS

For a better understanding of various embodiments of the present invention, reference will now be made by way of example only to the accompanying drawings in which:

FIG. 1 illustrates an apparatus;

FIG. 2 illustrates a first apparatus transmitting a signal to a second apparatus;

FIG. 3 illustrates a method of distance estimation;

FIG. 4 illustrates a schematic diagram of a transmitter;

FIG. 5 illustrates a schematic diagram of a receiver;

FIG. 5 illustrates a schematic block diagram of a first method;

FIG. 6 a illustrates a constellation diagram for Quadrature Phase Shift Keying;

FIG. 6 b illustrates a constellation diagram for Quadrature Phase Shift Keying, including value representing a measured data;

FIG. 7 illustrates a first signal being transmitted from a first apparatus to a second apparatus and a second signal being transmitted from the second apparatus to the first apparatus; and

FIG. 8 illustrates a schematic block diagram of a second method of distance estimation.

DETAILED DESCRIPTION OF VARIOUS EMBODIMENTS OF THE INVENTION

The Figures illustrate an apparatus 10, comprising: a receiver 18 configured to be enabled periodically to receive a signal, every first time; transformation circuitry 306 configured to transform a received signal, to determine data in the received signal; a comparator 316 configured to compare the determined data with reference data; and estimation circuitry 304 and 320 configured to use the difference between the determined data and the reference data to estimate a correction to a multiple of the first time, in order to determine a distance between the receiver and an origin of the signal.

FIG. 1 illustrates an apparatus 10. The apparatus 10 may, for example, be a portable electronic device. The apparatus 10 comprises a processor 12, a storage device 14, a transceiver 20, a user output device 22 and a user input device 24.

The processor 12 is connected to write to and read from the storage device 14. The storage device 14 may be a single memory unit or a plurality of memory units.

The processor 12 is connected to receive an input from the user input device 24 and may, for example, comprise a keypad and/or an audio input. The processor 12 is also connected to provide an output to the user output device 22. The user output device 22 is for conveying information to a user and may, for example, comprise a display or an audio output. The user input device 24 and the user output device 22 may be provided as a single unit, such as a touch screen display device.

The processor 12 is operable to receive an input from and provide an output to the radio transceiver 20. The radio transceiver 20 functions as a transmitter 16 and/or a receiver 18. The radio transceiver 20 may be configured to transmit and receive orthogonal frequency division multiplexed (OFDM) signals, such as Wireless Local Area Network (WLAN) 802.11a signals.

The receiver 18, transmitter 16 and the transceiver 20 are described below using a plurality of functional blocks. The receiver 18, transmitter 16 or the transceiver 20 may comprise a single integrated circuit or a set of integrated circuits (i.e. a chipset) for carrying out these functions. The integrated circuit(s) may comprise one or more hardwired, application-specific integrated circuits (ASICs) and/or one or more programmable processors for carrying out the functions using computer program instructions 26.

A chip or chipset for performing embodiments of the invention may be incorporated within a module. Such a module may be integrated within the apparatus 10, and/or may be separable from the apparatus 10.

It will also be appreciated by the skilled person that, although functions are described below as being performed by the transmitter 16, the receiver 18 or the transceiver 20, at least part of the functions may alternatively be carried out by the main processor 12 of the apparatus 10. In particular, time offset estimation and distance estimation may be performed by the processor 12.

The computer program instructions 26 may arrive at the apparatus 10 via an electromagnetic carrier signal or be copied from a physical entity 28 such as a computer program product, a memory device or a record medium such as a CD-ROM or DVD.

A storage device of the transmitter 16, the receiver 18 or the transceiver 20 or the storage device 12 of the apparatus 10 may store computer program instructions 26 that control the operation of the apparatus 10 when loaded into a processor. The computer program instructions 26 may provide the logic and routines that enables the apparatus to perform the methods illustrated in FIGS. 3 and 8.

The computer program instructions provide:

-   instructions for transforming a received signal, to determine data     in a received signal, wherein the signal is received periodically     every first time; -   instructions for comparing the determined data with reference data;     and -   instructions for using the difference between the determined data     and the reference data to estimate a correction to a multiple of the     first time, in order to determine a distance between the receiver     and an origin of the signal.

FIG. 2 illustrates a first apparatus 110 transmitting an OFDM signal 130 to a second apparatus 120. The first apparatus 110 and the second apparatus 120 may take the same form as the apparatus illustrated in FIG. 1.

FIG. 3 illustrates a flow diagram of a method for estimating the distance between the first apparatus 110 and the second apparatus 120 illustrated in FIG. 2.

At block 140 of FIG. 3, the first apparatus 110 transmits a signal 130 to the second apparatus 120.

FIG. 4 illustrates a functional schematic of the transmitter 16 used to transmit the signal 130. In this embodiment, the transmitter 16 is configured to transmit OFDM signals. The transmitter 16 comprises circuitry relating to serial-to-parallel conversion 204, symbol creation 206, transformation 208, digital-to-analogue conversion 212, 214, frequency up-conversion 216 and a symbol clock 220.

The processor 12 of the apparatus 10 is configured to output a serial bitstream 202 to the serial-to-parallel converter 204 of the transmitter 16. The serial bitstream 202 is data that is to be transmitted to the second apparatus 120. The serial-to-parallel converter 204 converts the serial bitstream to N parallel bitstreams 203 by interleaving the data on the serial bitstream 202. FIG. 4 shows the serial bitstream 202 being converted into four parallel bitstreams 203, illustrating one particular embodiment of the invention. In practice, the serial bitstream could be converted into any number of parallel bitstreams.

The N parallel bitstreams 203 are provided as an input to the symbol creation circuitry 206. The symbol creation circuitry 206 is configured to convert a segment of each the N parallel bitstreams into a symbol. Each symbol may comprise any number of bits. Symbols are created periodically by the symbol creation circuitry 206, according to a time basis set by the symbol clock 220.

N symbols are created by the symbol creation circuitry 206 and then set on a′bus 240 connecting the symbol creation circuitry 206 with the transformation circuitry 208. N different symbols are set on the bus 240 for a setting period T_(set) during every period T_(clock) of the symbol clock 220. The clock period T_(clock) may include a guard period T_(guard), which is a period in which no information is set on the bus 240, enabling the transformation circuitry 208 to clearly differentiate when the information being provided by the symbol creation circuitry 206 changes. If there is a guard period, T_(clock)=T_(set)+T_(guard).

It may be that the symbol creation circuitry 206 creates symbols using a modulation technique such as amplitude modulation or phase shift keying. An example of a suitable modulation technique is Quadrature Phase Shift Keying (QPSK). In QPSK, a bitstream is split into in-phase (I) and quadrature (Q) components. Each symbol consists of two bits. FIG. 6 a illustrates the position of each of the possible QPSK symbols ([0,1], [1,1], [1,0], [0,0]) on a constellation diagram as vector points 610, 620, 630 and 640.

The transformation circuitry 208 performs a transformation on the input symbols. An example of a suitable transformation is an inverse discrete Fourier transform, which changes the input signal from the frequency domain to the time domain. An inverse fast Fourier transform algorithm may be used to perform the inverse discrete Fourier transform. After an inverse discrete Fourier transform has been performed on the N input symbols, a signal is produced that includes an OFDM sub-carrier for each of the N input symbols.

The real part of each signal output by the transformation circuitry 208 is provided as an input to a digital-to-analogue converter (DAC) 212. Similarly, the imaginary part of each signal is provided to a DAC 214. The outputs from the DAC's 212, 214 are provided to a frequency upconverter 216, which changes the frequency of each signal from the baseband frequency to a frequency suitable for RF transmission. The frequency upconverter 216 provides an output to an antenna 218, which transmits the OFDM signals 130 as electromagnetic waves.

At block 150 of FIG. 3, the OFDM signal 130 is received at the antenna 322 of the second apparatus 120.

FIG. 5 illustrates a functional schematic of the receiver 18 of the second apparatus 120, which is configured to receive the OFDM signal 130. The receiver 18 comprises a reception symbol clock 308, a frequency down-converter 314, analogue-to-digital converters (ADC's) 310 and 312, transformation circuitry 306, symbol estimation circuitry 316, a parallel-to-serial conversion circuitry 302, time offset estimation circuitry 320 and distance estimation circuitry 321.

The reception symbol clock 308 periodically enables the antenna 322 to receive an OFDM signal. When the antenna 322 receives the OFDM signal 130 during an enablement period, it provides the signal as an input to a frequency down-converter 314. The frequency down-converter 314 reduces the frequency of the received OFDM signal and provides the real and imaginary parts of the signal to first and second ADC's 312 and 310. The first ADC 312 provides the real part of the signal to the transformation circuitry 306 (in digitized form) and the second ADC 310 provides the imaginary part (in digitized form).

At block 160 of FIG. 3, a transform is performed on the received signal to determine data.

The transformation circuitry 306 of the receiver performs an inverse transformation to that performed by the transformation circuitry 208 of the transmitter 16. For example, where the transformation circuitry 208 of the transmitter 16 is configured to perform an inverse discrete Fourier transform to convert the signal from the frequency domain to the time domain, the transformation circuitry 306 of the receiver 18 is configured to perform a discrete Fourier transform. A fast Fourier transform algorithm may be used to perform the discrete Fourier transform.

The transformation circuitry 306 transforms the input signal (including the real and imaginary parts) and outputs N data signals to symbol estimation circuitry 304, where each output data signal relates to a symbol.

At block 170 of FIG. 3, the determined data (corresponding to the received symbols) is compared with reference data to determine which symbols have been received.

The embodiment shown in FIG. 5 illustrates 4 lines of signals being provided to the symbol estimation circuitry 304, reflecting the four lines of symbols that were provided to the transformation circuitry 208 in the transmitter 16. The number of symbols recovered from the received OFDM signal following transformation depends upon the number of symbols inserted into the transmitted signal.

In ideal conditions, the data output by the transformation circuitry 306 of the receiver 18, which represents symbols, will correspond exactly on a constellation diagram with the symbols that were created by the symbol creation circuitry 206 of the transmitter 16. That is, if symbols corresponding to points 610, 620, 630 and 640 in FIG. 6 a (representing the symbols [0,1], [1,1], [0,0] and [1,0] respectively) were created by the symbol creation circuitry 206 of the transmitter 16, data corresponding to points 610, 620, 630 and 640 will be output by the transformation circuitry 306 of the receiver 18.

However, if conditions are not ideal, the data output by the transformation circuitry 306 will not correspond exactly with the vector points 610, 620, 630 and 640.

The symbol estimation circuitry 316 of the receiver 18 comprises a storage device 318 and a comparator 316. The storage device 318 stores reference data corresponding to all of the possible symbols that may be created by the symbol creation circuitry 206 of the transmitter 16. For instance, In the QPSK example illustrated in FIG. 6 a, reference data corresponding to symbols 610, 620, 630 and 640 is stored in the storage device 318.

The comparator 316 is configured to compare the reference data with the data determined by the transformation circuitry 306. In the ideal case, the comparator 316 is able to determine from the comparison that the determined data matches exactly with the stored reference data, and is therefore able to ascertain which symbols the determined data relates to. In a non-ideal case, the comparator 316 is configured to determine from the comparison which reference symbols the determined data is most likely to correspond with.

Non-ideal conditions arise if the symbol clock 220 of the transmitter 16 and the reception symbol clock 308 of the receiver 18 are not synchronized, resulting in the data determined by the transformation circuitry 306 of the receiver 18 being phase-offset from the reference data stored in the storage device 318.

Non-ideal conditions also arise if the symbol clocks 220, 308 are synchronized but the electromagnetic signal that is received by the antenna of the receiver 18 is not received at the same phase as the transmitted electromagnetic signal (that is, if the receiver 18 is not a distance corresponding to whole number of signal wavelengths away from the transmitter 16), resulting in the data determined by the transformation circuitry 306 of the receiver 18 being phase-offset from the reference data stored in the storage device 318.

FIG. 6 b illustrates a constellation diagram having a vector point 650 which relates to a portion of the determined data. The vector point 650 relates to the symbol [1,1], but is phase-offset from the ideal position of the symbol [1,1] given by point 620.

The comparator 316 is configured to compare the portion of determined data with the reference data stored in the storage device 318 and determine the closest ‘ideal point’ to the determined vector point 650. In this instance, the closest ideal point is the vector point 620, which corresponds to the symbol [1,1]. The comparator 316 therefore estimates that the portion of the determined data corresponds to the symbol [1,1].

Once the symbol estimation circuitry 304 has estimated which symbols correspond with the determined data, the estimated symbols are output to the parallel-to-serial converter 302. The parallel-to-serial converter 302 combines the N parallel bitstreams (in the illustrated case, N=4) to produce a serial bitstream 202, performing an inverse operation to that performed by the serial-to-parallel converter 204 of the transmitter 16. The serial bitstream 202 is output to the processor 12 of the apparatus for processing.

At block 180 of FIG. 3, the distance between the first apparatus 11 and the second apparatus 120 is estimated.

The distance d between the first apparatus 110 and the second apparatus 120 is given by the equation:

d=c×t _(tof)  (1)

where c is the speed of light and t_(tof) is the time of flight of the received signal.

Consider a situation where the reception symbol clock 308 operates with the same periodicity as the symbol clock 220 and is synchronized with the symbol clock 220. This can be achieved by the first apparatus 110 transmitting information relating to its clock to the second apparatus 120.

In this case, it can be assumed that the measured difference in phase between the transmitter 16 and the receiver 18 is due to the electromagnetic signal being received by the receiver 18 at a different phase to the transmitted electromagnetic signal.

A rough estimate of the time of flight of the received signal t_(tof-approx) can be made by counting the number of whole periods of time of the reception clock 308 (or, equivalently, the symbol clock 220) n that elapse between the transmission and the reception of the electromagnetic OFDM signal 130 and multiplying it by the period T_(clock) of the reception clock 220:

t _(tof-approx) =nT _(clock)  (2)

A rough estimation of the distance that separates the first apparatus 110 and the second apparatus 120 is therefore:

d _(approx) =c×nT _(clock)  (3)

However, this estimation of the distance can be improved by determining how the phase of the received signal differs from the phase of the transmitted signal.

The position of each vector point on the constellation diagram is described by the equation:

z=Re ^(iθ)  (4)

where z is the position of the vector point on the constellation diagram, R is the radius of the point from the intersection of the Inphase and Quadrature axes and θ is the angle between the vector and the Inphase axis.

The phase offset θ_(offset) between the measured vector point 650 and the ideal vector point 620 is given by:

θ_(offset)=θ₂−θ₁  (5)

where θ₂ is the phase of the determined vector point 650 and θ₁ is the phase of the ideal point 620, as measured from the Inphase axis of the constellation diagram (see FIG. 6 b).

The phase offset θ_(offset) can be used to make a correction to the n^(th) multiple of the symbol clock time T_(clock) and therefore also to make a correction to the rough estimate of the time of flight t_(tof-approx), by calculating a time offset, Δt, where:

$\begin{matrix} {{\Delta \; t} = \frac{\theta_{offset}}{\omega}} & (6) \end{matrix}$

where ω is the angular frequency of the determined vector point 650 i.e. the sub-carrier frequency for the detected symbol.

The time offset estimation circuitry 320 is configured to perform the calculation given in equation 6 after receiving the relevant phase offset information from the symbol estimation circuitry 304.

An improved estimation of the time of flight of the received signal, t_(tof-imp), can be found by adding the number of periods of time that have elapsed between transmission and reception of the electromagnetic signal to the average time offset, Δt:

t _(tof-imp) =nT+Δt  (7)

An improved calculation of the distance d may then be made using the improved estimation of the time of flight:

d _(est-imp) =c×t _(tof-imp)  (8)

where d_(est-imp) is the improved estimation of the distance between the first apparatus 110 to the second apparatus 120.

The distance estimation circuitry 321 is configured to perform the calculation given in equations 7 and 8 after receiving the relevant time offset information from the time offset estimation circuitry 320.

Optionally, the distance d_(est-imp) may be estimated multiple times (e.g. one for each sub-carrier) and averaged by the distance estimation circuitry 321 to reduce error.

FIG. 7 illustrates a first OFDM signal 730 being transmitted from a first apparatus 710 to a second apparatus 720, and a second signal 740 being transmitted from the second apparatus 720 to the first apparatus 710. The first and second apparatus 710 and 720 take the same form as the apparatus 10 described in FIG. 1.

In this embodiment, the first and second apparatuses 710 and 720 each comprise a transceiver 20 which has the functionality of the transmitter 16 and the receiver 18 described above. It should be recognized that the schematics of the transmitter 16 and the receiver 18 in FIGS. 4 and 5 illustrate the functions of the transceiver when transmitting and receiving. Each of the components/blocks in these schematics need not relate to a separate element in the transceiver 20. For example, the antenna 322 used for reception may be the same as the antenna 218 used for transmission.

In a transceiver 20, the symbol clock 220 of the transmitter 16 and the reception clock 308 of the receiver 18 are synchronized, have the same periodicity and may operate using the same clock source.

FIG. 8 illustrates a flow diagram of a method relating to the FIG. 7 embodiment of the invention. At block 810, the first apparatus 710 transmits the first signal 730 to the second apparatus 720. From the point at which the first symbol is created by the symbol creator 206, the first apparatus 710 begins to count the number n of symbol clock 220 periods of time T_(clock) that elapse.

At block 820 of FIG. 8, the second apparatus 720 receives the first signal 730 and waits for a period of time mT_(clock), and then transmits the second signal 740 to the first apparatus 710, where m is an integer and T_(clock) is the period of the symbol clock 220 in the first apparatus 710.

The first apparatus 710 receives the second signal 740 and follows the same processes as those discussed above in relation the reception of signal 130 in block 150 of FIG. 3.

At block 830 of FIG. 8, the transformation circuitry 306 of the receiver 18 performs a transform on the received second signal to determine data, in the same manner as that discussed above in relation block 160 of FIG. 3.

At block 840 of FIG. 8, the determined data (corresponding to the received symbols) is compared with reference data to determine which symbols have been received, in the same manner that described above in relation to block 170 of FIG. 3.

At block 850 of FIG. 8, the difference between the determined data and the reference data is used to estimate the distance between the first apparatus 710 and the second apparatus 720.

A time offset Δt for a particular symbol may calculated using equations 5 and 6, as discussed above. The time taken for the second apparatus 720 between receiving the first signal 730 and transmitting the second signal 740, which is equal to mT_(clock), is known to the first apparatus 710 meaning that advantageously, the first apparatus 710 is able to determine the combined time of flight of the first and second signals, T_(tof-12), using the equation:

t _(tof-12) =T _(clock)(n−m)+Δt  (9)

It may be that whenever the second apparatus 720 transmits a second signal 740 in response to the reception of a first signal 730, the time taken between the reception of the first signal 730 and the transmission of the second signal 740, mT_(clock), remains the same, and that this predetermined value is known the first apparatus 710. In a situation where the second apparatus 720 chooses to deviate from the predetermined value (e.g. because the transmission channel to be used is unavailable), it may include an indication of the value of m or mT_(clock) as data in the second signal 740 or in another signal transmitted to the first apparatus 710.

It follows from equation (9) that the estimated distance d_(est) from the first apparatus 710 to the second apparatus 720 is:

$\begin{matrix} {d_{est} = \frac{c\left\lbrack {{T_{clock}\left( {n - m} \right)} + {\Delta \; t}} \right\rbrack}{2}} & (10) \end{matrix}$

Optionally, the distance d_(est) may be estimated multiple times (once for each sub-carrier) and averaged by the distance estimation circuitry 321 to reduce error.

In some embodiments, the estimated distance d_(est) between the first apparatus 710 and the second apparatus 720 may be determined at both the first apparatus 710 and the second apparatus 720.

In one embodiment, a third signal may be transmitted from the first apparatus 710 to the second apparatus 720. The first apparatus 710 may use the first and second signals to determine the estimated distance d_(est) and the second apparatus 720 may use the second and third signals to determine the estimated distance d_(est).

Alternatively, a third signal may be transmitted from the second apparatus 720 to the second apparatus 710 and a fourth signal may be transmitted from the first apparatus to the second apparatus 720. In this example, the first apparatus 710 may use the first and second signals to determine the estimated distance d_(est) and the second apparatus 720 may use the third and fourth signals to determine the estimated distance d_(est).

The second apparatus 720 may, for example, transmit its distance estimation to the first apparatus 710 to enable the first apparatus 710 to produce an averaged distance estimation using the estimated value(s) calculated at the second apparatus 720.

At least portions of the blocks illustrated in FIGS. 3 and 8 may represent steps in a method and/or sections of code in the computer program 26. The illustration of a particular order to the blocks does not necessarily imply that there is a required or preferred order for the blocks and the order and arrangement of the block may be varied.

Although embodiments of the present invention have been described in the preceding paragraphs with reference to various examples, it should be appreciated that modifications to the examples given can be made without departing from the scope of the invention as claimed.

Features described in the preceding description may be used in combinations other than the combinations explicitly described.

Whilst endeavoring in the foregoing specification to draw attention to those features of the invention believed to be of particular importance it should be understood that the Applicant claims protection in respect of any patentable feature or combination of features hereinbefore referred to and/or shown in the drawings whether or not particular emphasis has been placed thereon. 

1. A method, comprising: enabling reception of an orthogonal frequency division multiplexed signal at a receiver, every first time, the orthogonal frequency division multiplexed signal comprising a plurality of sub-carriers; transforming a received orthogonal frequency division multiplexed signal in order to determine data from the sub-carriers of the received orthogonal frequency divisional multiplexed signal; comparing the data determined from the sub-carriers with reference data; and using the difference between the data determined from the sub-carriers and the reference data to estimate a correction to a multiple of the first time in order to determine a distance between the receiver and an origin of the orthogonal frequency division multiplexed signal.
 2. A method as claimed in claim 1, wherein the distance is determined by determining the time of flight of the orthogonal frequency divisional multiplexed signal, and the time of flight of the orthogonal frequency divisional multiplexed signal is determined using a multiple of the first time and the correction to the multiple of the first time.
 3. A method as claimed in claim 1, wherein the data determined from the sub-carriers is compared with the reference data to determine phase differences between the phase of the sub-carriers when transmitted and the phase of the sub-carriers when received, and the determined phase differences are used to estimate the correction to the multiple of the first time in order to determine the distance between the receiver and the origin of the orthogonal frequency divisional multiplexed signal.
 4. A method as claimed in claim 3, wherein the reference data and the data determined from the sub-carriers is the same if no phase difference is determined.
 5. A method as claimed in claim 1, wherein the orthogonal frequency division multiplexed signal is transformed from the frequency domain to the time domain.
 6. A method as claimed in claim 1, wherein the distance between the receiver and the origin of the orthogonal frequency division multiplexed signal is measured a plurality of times, each time using a different sub-carrier of the orthogonal frequency division multiplexed signal.
 7. A method as claimed in claim 1, wherein the receiver is a transceiver, further comprising the transceiver transmitting a further signal to a further transceiver, and, subsequently, the further transceiver transmitting the orthogonal frequency divisional multiplexed signal to the transceiver.
 8. A method as claimed in claim 7, wherein the time taken by the further transceiver between receiving the further signal and transmitting the signal to the transceiver is a multiple of the first time.
 9. A method as claimed in claim 8, wherein the distance is determined by determining the time of flight of the orthogonal frequency divisional multiplexed signal, and the time of flight of the orthogonal frequency divisional multiplexed signal is determined using a multiple of the first time, the time taken by the further transceiver and the correction to the multiple of the first time.
 10. Apparatus, comprising: transformation circuitry configured to transform a received orthogonal frequency division multiplexed signal comprising a plurality of sub-carriers, in order to determine data from the sub-carriers of the received orthogonal frequency division multiplexed signal; the orthogonal frequency divisional multiplexed signal being received periodically every first time; a comparator configured to compare the data determined from the sub-carriers with reference data; and estimation circuitry configured to use the difference between the data determined from the sub-carriers and the reference data to estimate a correction to a multiple of the first time, in order to determine a distance between the receiver and an origin of the orthogonal frequency division multiplexed signal.
 11. Apparatus as claimed in claim 10, wherein the estimation circuitry is configured to determine the distance by determining the time of flight of the orthogonal frequency division multiplexed signal, and the time of flight of the orthogonal frequency division multiplexed signal is determined using a multiple of the first time and the correction to the multiple of the first time.
 12. Apparatus as claimed in claim 10, wherein the comparator is configured to compare the data determined from the sub-carriers with the reference data to determine phase differences between the phase of the sub-carriers when transmitted and the phase of the sub-carriers when received, and the estimation circuitry is configured to use the determined phase differences to estimate the correction to the multiple of the first time, in order to determine the distance between the receiver and the origin of the orthogonal frequency division multiplexed signal.
 13. Apparatus as claimed in claim 12, wherein the reference data and the data determined from the sub-carriers is the same if no phase difference is determined.
 14. Apparatus as claimed in claim 10, wherein the transformer is configured to transform the orthogonal frequency division multiplexed signal from the frequency domain to the time domain.
 15. Apparatus as claimed in claim 10, wherein the estimation circuitry is configured to measure the distance between the receiver and the origin of the frequency division multiplexed signal is measured a plurality of times, each time using a different sub-carrier of the orthogonal frequency division multiplexed signal.
 16. (canceled)
 17. (canceled)
 18. (canceled)
 19. (canceled)
 20. (canceled)
 21. (canceled)
 22. A computer-readable storage medium storing a computer program comprising computer program instructions that, when executed by at least one processor, cause an apparatus to perform at least the following: for transforming a received orthogonal frequency division multiplexed signal comprising a plurality of sub-carriers, to determine data in a received orthogonal frequency division multiplexed signal, wherein the orthogonal frequency division multiplexed signal is received periodically every first time; comparing the data determined from the sub-carriers with reference data; and using the difference between the data determined from the sub-carriers and the reference data to estimate a correction to a multiple of the first time, in order to determine a distance between the receiver and an origin of the orthogonal frequency division multiplexed signal.
 23. A computer-readable storage medium as claimed in claim 22, wherein the distance is determined by determining the time of flight of the orthogonal frequency division multiplexed signal is determined using a multiple of the first time and the correction to the multiple of the first time.
 24. A computer-readable storage medium as claimed in claim 22, wherein the data determined from the sub-carriers is compared with the reference data to determine phase differences between the phase of the sub-carriers when transmitted and the phase of the subcarriers when received, and the determined phase differences are used to estimate the correction to the multiple of the first time in order to determine the distance between the receiver and an origin of the signal.
 25. A computer-readable storage medium as claimed in claim 24, wherein the reference data and the data determined from the sub-carriers is the same if no phase difference is determined.
 26. (canceled)
 27. (canceled)
 28. Apparatus as claimed in claim 10, further comprising a receiver configured to be enabled periodically to receive an orthogonal frequency 